Thermal printing head with diffused printing elements



June 2, 1970 R CQCADY; JR 3,515,850

' THERMAL PRINTING HEAD wmx DIFFUSED PRINTING ELEMENTS Filed 00% 2, 1967 v 5 Sheets-Sheet 1 INVENTOR HIS ATTORNEYS RICHARD ClCADY, JR.

June 2, 1970 v R. c. CADY, JR 3, 0

THERMAL PRINTING HEAD WIT H DIFFUSED PRINTING ELEMENTS Filed Oct. 2, 1967 I 5 Sheets-Sheet 2 INVENTOR RICHARD c. CADY, JR

HIS ATTORNEYS Jungz, 1970 RQCA JR 3,515,850

THERMAL PRINTING HEAD WITH DIFFUSED PRINTING ELEMEN'IES Filed 051:. 2, 1967 5 Sheets-Sheet 4 FIG.5 4s 49 INVENTOR HIS ATTORNEYS RICHARD C. CADY, JR.

June 2, 1970 I R. c. CADY. JR 3,515,850-

' THERMAL PRINTING HEAD WITH DIFFUSED PRINTING ELEMENTS Filed Oct. 2, 1967 5 Sheets-Sheet 5 FIG.6

HIS ATTORNEYS RICHARD C. CADY, JR.

United States Patent 3,515,850 THERMAL PRINTING HEAD WITH DIFFUSED PRINTING ELEMENTS Richard C. Cady, In, Dayton, Ohio, assignor to The National Cash Register Company, Dayton, Ohio, a corporation of Maryland Filed Oct. 2, 1967, Ser. No. 672,319 Int. Cl. G01d /10; H051) 3/02; H041 15/34 US. Cl. 219216 4 Claims ABSTRACT OF THE DISCLOSURE A thermal printing head which includes anumber of semi-conductor printing elements diffused into a semi-conductor body for marking a thermally-sensitive record material is disclosed. The diffused thermal printing elements are arranged into a matrix, and matrix selection is provided by an interconnection network consisting of first level electrodes which pass through a dielectric insulating layer to a second level electrical selection conductor. Isolation elements which are also diffused into the semi-conductor body are interconnected with the diffused printing elements. The isolation elements may be a semi-conductor diode, a silicon controlled rectifier, or a semi-conductor threshold device.

A protective overcoating layer is deposited over the entire structure including the semi-conductor printing element and the semi-conductor isolation element. One embodiment of the present invention employs a protective overcoating layer of a material having a substantial thermal conductivity and a substantial electrical resistivity which is shaped to provide a raised area over each of the resistive printing elements so as to provide a plurality of selectively heat-concentratable raised areas for printing.

Background of the invention Prior thermal printing devices are constructed of a number of individual printing elements which must be aligned in a co-planar array. The number of external connections that are required for such thermal printing heads is fairly large, and, in addition, although a relatively hard material, tin oxide, is employed for the resistive elements, abrasion is a problem with this type of thermal printing head.

The diffused thermal printing matrix structure of the present invention eliminates co-planar alignment requirements, reduces the number of electrical connections that are required by an order of magnitude, and provides an abrasion-resistive overcoating layer.

Brief description of the drawings FIG. 1 is a sectional perspective view, taken along the line 11 of FIG. 5, which shows a single diffused printing element and its associated isolation element.

FIG. 2 is an electrical schematic showing the electrical interconnections for a thermal printing matrix of twentyfive printing elements.

Description of the preferred embodiment Referring to FIG. 1, the printing head shown therein includes a substrate 15, which is single-crystal sapphire,

3,515,850 Patented June 2, 1970 upon which is deposited, by standard epitaxial deposition techniques, a layer of semi-conductive material 9, such as single-crystal silicon, of N-type conductivity. A heatconductive layer 14, such as aluminum, may be secured to the substrate 15 to conduct heat away from the substrate. The semi-conductive layer 9 has a thickness in the range between 0.0001 inch and 0.0005 inch. The semiconductive layer 9 is then selectively etched off of the substrate 15 to form a matrix of isolated, rectangular islands or areas 16 (FIGS. 4 t0 6). The printing elements 10 of 'FIG. 2, comprising a resistive member 11 and an isolation diode 12, are subsequently formed on the islands 16. The islands 16 are formed to provide electrical and thermal isolation between the printing elements 10.

The following steps are performed, using photolithographic, etching, and diffusion techniques that are standard in the art. The islands 16 are delineated and etched, and diffused P-type regions 17 and 18 are formed therein (see FIGS. 3 to 5). Then, the islands 16 are again delineated and etched, and a diffused N+-type region 19 is formed therein. The islands 16 are again delineated, and the silicon dioxide layer 20 (FIG. 3), which was formed during and after each of the prior diffusion steps, is etched away in certain areas to form holes 21 to 24 therein, through which ohmic contact is made to the underlying semi-conductor material. The resistive member 11 of FIG. 2 is the P-type region 17. The isolation diode 12 is formed of the material of the island 16, the P-type region 18 is the diode anode, and the N -type region 19 is ohmic contact region for the diode cathode.

The foregoing processing steps largely determine the electrical properties of the printing elements 10. The N-type single-crystal silicon layer formed into the islands 16 largely determines the reverse-breakdown voltage of the isolation diodes .12 and is chosen with the proper N-type impurity concentration so as to provide a satisfactory diode yield. The N -type regions 19 are formed so that non-rectifying electrical contact can be made to the isolation diode by means of an electrical conductor. The proximity of an N+-type region 19 to the anode region 18 of the isolation diode strongly affects the diode forward resistance. The diffusion of impurities into the P-type regions 17 and the geometry of these regions may both be varied to obtain the desired resistance value for the resistive member 11. Additionally, the geometry of the P-type regions 18, which form the anodes of the isolation diodes 12, may be varied, and this effects the reverse leakage of the isolation diodes 12.

The following steps in the manufacture of the printing head are then performed. First, or bottom level, electrical conductors 25, 26, 28, 29, and 30 are formed in the holes 21 to 24 in the silicon oxide layer 20. The electrical conductor 25 extends across all of the printing elements 10 (see FIG. -6) and constitutes the common drive line for them. The electrical conductor 25 makes ohmic contact with the P-type region 18 forming the anode of the isolation diode 12. The electrical conductor 26 electrically interconnects the N -type region 19 and one end of the P-type region 17 which forms the resistive member 11. The electrical conductor 27 electrically interconnects the other end of the P-type region 17 with a second, or top level, electrical conductor 31, which is formed later. The electrical conductors 28, 29, and 30 extend across the printing elements 10 (see FIG. 6) and form the energization or drive lines for them. The electrical conductor 28 serves as an energization line for the printing element 10, shown in FIG. 3, and similarly-located printing elements 10 in each matrix, as shown in FIG. 6. The electrical conductors 29 and 30 serve as energization lines for other printing elements in each matrix, as indicated in FIG. 6'. Other second level electrical conductors 33 and 34 are provided. The conductor 33 provides a means for supplying energizing current to the first level electrical conductor 29, and the conductor 34 connects the first level conductor 30 to a first level heating resistor.

The first level electrical conductors 25 to 30 in the described embodiment are formed as molybdenum-gold sandwich structures. These structures comprise a layer of evaporated molybdenum, a layer of gold, and a second layer of molybdenum. The molybdenum adheres tightly to the silicon dioxide layer and other dielectric material. The conductors to may also comprise vacuum-deposited aluminum, which also strongly adheres to the silicon dioxide layer 20 and other dielectric material.

The interconnection pattern for the first level electrical conductors 25 to 30 is delineated by standard photolithographic techniques. The character common drive conductors (such as the conductor 25)"; the element drive lines (such as the conductors 28, 29, and 30), and the connection between the isolation diode N+-type region 19 and one end of the P-type region 17 (forming a re sistive member 11) are formed. The first level interconnection pattern is shown in FIG. 6.

A dielectric insulating layer (FIG. 3) is then deposited over the entire surface of the printing head to electrically isolate the first level electrical conductors 25 to 30 from the second level electrical conductors 31, 33, and 34, and the other second level electrical conductors. The material of the layer 45 should be capable of forming a strong bond with the molybdenum layer of the first level electrical conductors, its temperature coefiicient of expansion should match that of the silicon islands 16 reasonably well, and it should form a strong bond to the silicon dioxide layer 20. Special glasses are commercially available which meet the foregoing criteria. The glass may be deposited by radio frequency sputtering. The layer 45 should have suflicient breakdown voltage to withstand the voltage (about 20 to 25 volts) applied across a printing element 10, and it must be free of pinholes.

Next, the first level electrical conductors 25 to 30 are exposed in those areas where electrical contact is desired to be made between a first and a second level electrical conductor. This is accomplished by delineating the areas by photolithographic techniques followed by selective etching, or by masking the areas so that the material of the insulating layer 45 will not adhere to them.

The second level interconnection pattern, which comprises the electrical conductors 31, 33, and 34, is deposited and delineated. The electrical conductors, such as the conductors 31, of each printing element connects the printing element drive conductors, such as the first level conductors 28, 29, and 30, to the P-type regions 17 which form the heating resistive member 11. The electrical conductors such as the conductor 35 (FIG. 6) connect the character common conductors, such as the conductors 25 and 39, together and furnish an electrical path to external contact pads, such as the pad 47. The electrical conductors such as the conductors 34, 36, and 37 furnish an electrical path from the printing element drive conductors, such as the first level conductors 28, 29, and 30,

to external contact pads, such as the pads 48, 49, and 50. The conductors of the second level interconnection pattern are one of the same materials as the conductors of the first level interconnection pattern and are deposited and delineated in a similar manner.

A protective layer 51 (FIG. 3) is deposited over the entire surface of the printing head except for the external contact pads, such as the pads 47 to 50. The protective layer 51 should have high thermal conductivity and high electrical resistivity. It should also offer good resistance to abrasion and be capable of withstanding repeated, rapid (cycle time about 25 milliseconds) cycling from room temperature to about 300 degrees centigrade. The metal-oxide-ceramics fulfill these criteria, and preferred material for the protective coating 51 is beryllium oxide (BeO) and aluminum oxide (A1 0 Beryllium oxide is particularly suited to function as a protective coating because of its hardness and its very high thermal conductivity, and because its coeflicient of thermal expansion is near' to that of silicon (10.0 microinches per inch per degree centigrade for beryllium oxide and 4.68 microinches per inch per degree centigrade for silicon). The protective layer 51 is deposited by standard radio frequency sputtering techniques.

Finally, as much of the protective layer 51 as is possible or practical is removed from between adjacent printing elements 10, as indicated by the depression at 52 in FIG. 3. These depressions help to produce localized high temperature areas that are confined to the printing elements 10 and help to reduce mutual interaction between adjacent printing elements 10. These depressions may be made by a photolithographic selective etching technique. The depressions may also be made during the process of depositing the protective layer 51 using a shadow-masking technique in which a wire grid, for example, is placed between the source of coating material and the printing head and casts a shadow on the printing head where less material will be deposited than where no shadow is cast.

What is claimed is:

1. A thermal printing device comprising:

(a) an electrically insulating substrate;

(b) a body of semi-conductive material deposited on a supporting surface of the substrate;

(c) a plurality of heat-generating elements of a material having a substantial electrical resistivity, the heat-generating elements being diffused into the body of semi-conductive material so as to define an information pattern;

(d) a plurality of electrical isolation elements, each electrical isolation element being associated with one of the heat-generating elements, each electrical isolation element being diffused into the body of semiconductive material in the vicinity of its heat-generating element, and each electrical isolation element having at least a first and a second terminal, the first terminal of each electrical isolation element being electrically connected to its associated heat-generating element, the electrical isolation elements being constructed to initially have a high impedance state with respect to the first and second terminals thereof when the voltage across the first and second terminals is at a first predetermined value, and to have a low impedance state with respect to the first and second terminals thereof when the voltage across the first and second terminals is at a second predetermined value;

(e) a first insulating layer of an electrically insulating material which overlies each heat-generating element and each electrical isolation element;

(f) at least one elongated electrical common conductor on the first insulating layer electrically connected to the second terminals of a plurality of electrical isolation elements in common through an opening in the first insulating layer;

(g) a first electrical connecting conductor provided for each heat-generating element on the first insulating layer that electrically connects the first terminal of each electrical isolation element to one portion of a heat-generating element through openings in the first insulating layer;

(h) a second electrical connecting conductor provided for each heat-generating element in electrical contact with another portion of a heat-generating element;

(i) a plurality of elongated electrical selection conductors on the first insulating layer, a predetermined one of said elongated electrical selection conductors being provided for the energization of a heatgenerating element associated therewith;

(j) a second insulating layer of an electrically insulating material which overlies the elongated electrical common conductor, the first electrical connecting conductors, and the elongated electrical selection conductors;

(k) and a third electrical connecting conductor provided for each heat-generating element on the second insulating layer that electrically connects each predetermined one of said elongated electrical selection conductors through an opening in the second insulating layer to its associated heat-generating element by way of the second electrical connecting conductor.

2. A device as in claim 1 further comprising a protective layer of an abrasion-resistive material having a substantial thermal conductivity and suflicient electrical resistivity to prevent electrical shorting of the heat-generat- References Cited UNITED STATES PATENTS 3,323,241 6/1967 Blair et a1. 219543 X 3,170,813 2/1965 Duncan et a1. 117-93.1 X 3,350,222 10/1967 Ames et a1. 117--2l5 X 3,415,680 12/1968 Perri et al. 1l7215 X JOSEPH V. TRUHE, Primary Examiner P. W. GOWDEY, Assistant Examiner US. Cl. X.R. 

